1. Field of the Invention
The present invention relates to an electrophotographic apparatus and an image forming method for reverse developing using an electrophotographic photosensitive member. More particularly, the present invention relates to an electrophotographic apparatus and an image forming method which require no idle rotation at the start of a copying operation, make high speed processing possible, suppress the generation of negative or positive ghosting, and suppress variations in electric characteristics and in image densities between first and second copying cycles, thereby enabling images of high quality to be prepared quickly and simply.
2. Description of the Related Art
An electrophotographic process invented by C. F. Carlson has been widely used in recent years not only in the fields of copying machines but also in various fields including the fields of printers and fax machines because of its excellent immediate processing ability and ability to prepare images having high quality and excellent storage stability. The electrophotographic process fundamentally comprises an image forming process and an initializing process. The image forming process includes uniformly charging an electrophotographic photosensitive member, forming an electrostatic latent image on the surface of the electrophotographic photosensitive member by image exposure corresponding to the original, making the electrostatic latent image visible by using a toner to form an image, transferring the toner image to a paper (through an intermediate transfer member in some cases), and fixing the image. The initializing process includes a step of removing the toner and charges remaining on the surface of the electrophotographic photosensitive member because the electrophotographic photosensitive member is repeatedly used.
Organic type photoconductive materials, which are non-polluting and have the advantages such as easy film formation and easy production, have been used in recent years as electrophotographic photosensitive members in place of inorganic type photoconductive materials such as selenium, arsenic/selenium alloy, cadmium sulfate, zinc oxide, and the like which have been conventionally used. Electrophotographic photosensitive members can be classified into a single layer type and a laminate type. Among these types, a laminate type photosensitive material comprising a charge generating layer and a charge transporting layer, which are laminated on an electroconductive support, has high sensitivity, can be selected from a wide variety of materials, is safe, possesses high produce ability, and is comparatively advantageous in terms of costs. Therefore, such a laminate type photosensitive material is very popularly used for the electrophotographic photosensitive member and is produced in a large amount at present.
Recently, the technology of digitally forming images, by which images of higher quality are obtained and by which storage and free editing of the input image are made possible, has come into wide use. Although devices for digitally forming images are limited to portions of color laser copiers, laser printers and LED printers which are output devices for word processors and personal computers, and the like, these digital systems have rapidly come to be used widely in the fields of ordinary copying machines which form images by analog processing.
When an image is formed digitally, image information is input into an electrophotographic photosensitive member after electric signals have been converted into optical signals in the case of directly using computer information which is electric signals, or, in the case of using information of an original, after reading information of the original as optical information, then converting the optical information into digital electric signals once, and then converting these digital electric signals again to optical signals. The image information is input into the electrophotographic photosensitive member as optical signals. For the input of such optical signals, laser light or LED light is usually used. At present, light: which is most frequently used for the input of the optical signals is near infrared light of an oscillation wavelength of 780 nm or 660 nm, or light of a long wavelength closing to these wavelengths.
The characteristic firstly required of an electrophotographic photosensitive member used when forming images digitally is sensitivity to light of these long wavelengths. In view of this, selection and application of various kinds of materials have been studied to obtain an electrophotographic photosensitive member having the above characteristic.
Among these various materials, phthalocyanine compounds have been widely studied and used in practice because their synthesis is relatively simple and many phthalocyanine compounds exhibit sensitivity to light of a long wavelength.
For example, a photosensitive material using titanyl phthalocyanine is disclosed in Japanese Patent Application Publication (JP-B) No. 5-55860, a photosensitive material using .beta.-type indium phthalocyanine is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 59-155351, a photosensitive material using X-type non-metal phthalocyanine is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2-233769, and a photosensitive material using vanadyloxy phthalocyanine is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 61-28557.
In the case of digital formation of an image, a so-called reverse developing method in which a toner is made to adhere to portions irradiated with light to form an image, is often used to efficiently utilize light or to improve the resolution. In this reverse developing method, dark potential portions become white background portions and light potential portions become black background portions (image line portions).
An electrophotographic photosensitive member, for which transfer of an image has been completed, is subjected to the above-mentioned initializing process because it must be used for the next image formation. As a charge-removing method in this process, a method utilizing AC corona discharge, a method utilizing light, and the like are known. Among these charge-removing methods, a charge removing method using light is often used because this method can be performed using a simple apparatus and is not accompanied by the generation of harmful gasses such as ozone and the like which are generated when using AC corona discharge.
The present inventors formed an image by an image forming process utilizing the reverse developing method using a laminate-type electrophotographic photosensitive member comprising a charge generating layer containing a phthalocyanine compound. However, they found that this image forming method had a drawback in that electrons tended to remain in the charge generating layer after holes were first injected into the laminate-type electrophotographic photosensitive member, and the electrons acted as a kind of memory causing variations in potential.
The fundamentals of this phenomenon are assumed to be that the electrons remaining in the charge generating layer advance for some reason to the boundary between the charge generating layer and the charge transporting layer, thereby reducing a barrier height for injecting holes in a vicinity of the boundary. In fact, when using a laminate-type electrophotographic photosensitive member which comprises a charge generating layer containing a phthalocyanine compound, the potential of a portion exposed by light in the previous exposure cycle is higher than that of the surrounding portions within an exposed area in this exposure cycle because of the difference due to the presence or lack of exposure in the previous exposure cycle, whereby a so-called negative ghosting phenomenon occurs. Or, because the sensitivity of a portion exposed by light in the previous cycle is apparently higher, a so-called positive ghosting phenomenon, in which the portion exposed by light in the previous cycle protrudes as a black portion, is marked when forming an image whose entire surface is uniform in this exposure cycle.
There is the potential for such problems to occur in the case of using a laminate-type electrophotographic photosensitive member comprising a charge generating layer containing a phthalocyanine compound in an image forming process utilizing the reverse developing method.
Currently, in order to avoid such problems, an image forming process by a first rotation of an electrophotographic photosensitive member, in which the charging potential is decreased, is not used for actual image formation (idle rotation), but image forming processes by the second and subsequent rotations of the electrophotographic photosensitive member, in which the charging potential is stabilized, are used for actual image formation. In the case of a conventional reverse developing-type printer which has a relatively slow copying speed (for example, 10 or fewer A4 size sheets per minute), the above problems are not conspicuously exhibited since the charging device has a sufficient ability to control the charge. Further, even an image forming process, in which the first rotation of the electrophotographic photosensitive member is idle, is used without any particular trouble because the transfer of data from a computer and the like requires some time, and the like. However, when an original is directly copied using a digital copier or the like with a high copying speed, there is a problem that the image forming process, in which the first rotation of the electrophotographic photosensitive member is idle, presents a great obstacle to high speed operation.
In light of this situation, there is a demand for the development of an electrophotographic apparatus and an image forming method by which an image can be formed even from the first rotation of a laminate-type electrophotographic photosensitive member.